Since the early 20th century, experimentation has been conducted with radio waves as a means to detect distant targets of interest. Radar systems have developed from simple detection and ranging systems to essential systems on board aircraft and ships, and more recently to highly sophisticated space surveillance and imaging systems. Radar technology has followed, and in many cases driven, the advances in high-frequency electronic systems, digital processing, and computing. Most radar systems employ distinct but collocated transmitter and receiver electronics, as is explained in elementary radar systems textbooks. A modern aircraft or ship may have multiple radar systems of this type, for detecting and ranging airborne vehicles, marine vessels, and/or weather phenomena. The development of bistatic radar systems, such that the radar transmitter and radar receiver are not collocated, has attracted some attention. Early use of radio wave detection of aircraft, for example, has included the implementation of bistatic reflections of a radio broadcast from a stationary transmitter to a distant receiver as a means to indicate the presence of a reflective target.
As radar technology has improved, radar based systems have been designed and implemented to provide critical imaging information regarding a target object or target area, particularly in scenarios where visible detection or visible images are not available or are not satisfactory. Signal parameters such as amplitude, time-delay, carrier-frequency, and modulation type are known to affect the performance of simple radar systems and advanced radar based imagery systems using synthetic aperture radar (SAR) techniques. In SAR systems, the motion of the platform hosting the radar transmitter is used to synthesize a much larger antenna aperture, consequently resulting in a higher resolution than is possible with the smaller physical aperture used in typical radar systems. The characteristics or parameters of radar signals that are reflected from a target object can be employed to provide imagery of the target. Because these images are generated from radio frequency (RF) waveforms as opposed to visible light, radar images can typically be obtained in poor weather or when the target is obscured by foliage, fog, or cloud cover.
In basic terms, SAR systems employ modulated pulse Doppler radar signals. Reflected signals from suitable radio-wave reflective targets can be processed to create a radar image that can often be distinct from an image obtained from a visible light based system. A SAR system typically uses the monostatic return from a target, which requires the radar receiver to be collocated, or nearly collocated, with the radar transmitter. As such, the SAR system can be located on a single platform in motion, such as an aircraft or orbiting satellite. In a bistatic or multistatic radar systems, the radar transmitter and the one or more radar receivers can be significantly separated in space, such that they can be located on separate and distinct platforms.
Important to the function of bistatic and multistatic systems is the variation in the target radar cross section (RCS) when computed for electromagnetic scattering at angles other than that obtained in monostatic radar systems. RCS is a parameter that characterizes the relative strength of the radar backscatter signal, and is a complex function of radio wave frequency, target geometry, electromagnetic scattering principles, and physical composition. As an example, objects with low RCS in a monostatic radar system often have high RCS when viewed as a bistatic or multistatic target by a radar receiver displaced by some significant angle with respect to the radar illumination.
The performance of typical SAR systems can be characterized by examining an ambiguity function of the transmitted radar signal. The ambiguity function of the radar signal is related to the autocorrelation of the signal as a function of system parameters, time delay, and Doppler frequency shift. Ideally, the ambiguity function can be plotted as a narrow spike centered at the origin, with limited energy content along both the time and Doppler axis. Errors in interpreting the radar signal parameters in the pulsed radar signals, as reflected from the target object or terrain, can result in artifacts and degraded resolution that can affect the processed radar image. Radar signals, including linear frequency modulated (FM) chirp pulse trains employed in SAR systems, may have limited bandwidth and time duration, such that the fundamental radar system performance can be compromised. The critical parameter of time-bandwidth product (TW) for a linear frequency modulated chirp is constrained by radar system design factors, such as ambiguous range, peak pulse power, and coherent bandwidth of the RF electronics. Accordingly, radar images may be generated without significant clarity or resolution.